President Petro Poroshenko declared an end to a cease-fire in eastern Ukraine late Monday and the beginning of a new attack to "liberate our land" from pro-Russia rebels, ignoring pressure from Moscow to extend the 10-day-old truce and dealing a blow to hopes for talks.

The decision to use the army represents a gamble by Mr. Poroshenko that Russia won't send in its troops, massed for months on the other side of the border, and that Kiev's ragtag forces can oust the increasingly well-armed militants who have seized...